Multiple WhatsApp users in China reported experiencing intermittent outages from Monday night Beijing time. By Tuesday morning, users had taken to social networks such as Twitter to report that photos as well as audio clips — a favoured format in the country — were not being delivered.

WhatsApp is not responsible for the blockage, according to a person familiar with the matter. The company declined to comment.

“The Chinese authorities want to be able to monitor all communication on the internet,” said Charlie Smith, a cofounder of GreatFire.org, which tracks blockages. Smith uses a pseudonym for fear of reprisals by Chinese authorities. “By blocking WhatsApp, they limit the choices that Chinese have to send private and encrypted communications and force more and more users to adopt WeChat as their messaging app.”

While WhatsApp messages are encrypted, WeChat is unencrypted and highly censored, Smith said. On WeChat, a hugely popular messaging app run by China’s Tencent Holdings, people are asked to use their real names. “This is part of the censorship master plan,” Smith said.

Chinese authorities have been ramping up censorship of social media and messaging platforms as the government prepares for the 19th Communist Party Congress — a politically sensitive event that is likely to consolidate President Xi Jinping’s authority.

Some users have begun relying on virtual private networks, a technology that allows users to route their data overseas, to get around the block — although China’s government has begun cracking down on these tools lately.